Temperature Stable MEMS Resonator

Abstract

One embodiment of the present invention sets forth a method for decreasing a temperature coefficient of frequency (TCF) of a MEMS resonator. The method comprises lithographically defining slots in the MEMS resonator beams and filling the slots with oxide. By growing oxide within the slots, the amount of oxide growth on the outside surfaces of the MEMS resonator may be reduced. Furthermore, by situating the slots in the areas of large flexural stresses, the contribution of the embedded oxide to the overall TCF of the MEMS resonator is increased, and the total amount of oxide needed to decrease the overall TCF of the MEMS resonator to a particular target value is reduced. As a result, the TCF of the MEMS resonator may be reduced in a manner that is more effective relative to prior art approaches.

1 . A microelectromechanical system (MEMS) resonator with a reduced magnitude of thermal coefficient of frequency (TCF) and configured to generate a reference signal, the MEMS resonator comprising:
 a base element; and   at least one resonating element coupled to the base element that includes one or more slots filled with a compensating material,   wherein a temperature coefficient of Young's Modulus (TCE) of the compensating material has a sign opposite to a TCE of a material comprising the at least one resonating element.   
   
   
       2 . The MEMS resonator of  claim 1 , wherein the one or more slots are positioned in a region of the at least one resonating element subject to larger stresses relative to other regions of the at least one resonating element. 
   
   
       3 . The MEMS resonator of  claim 2 , wherein the at least one resonating element resonates via an extensional and/or flexural mode. 
   
   
       4 . The MEMS resonator of  claim 3 , wherein a cross section of the at least one resonating element that is perpendicular to the extensional and/or flexural mode comprises approximately 40% of the compensating material. 
   
   
       5 . The MEMS resonator of  claim 2 , wherein the stresses decrease from an outside sidewall of the at least one resonating element to the center of the at least one resonating element, and a region proximate to the outside sidewall of the at least one resonating element is the region subject to the relatively larger stresses. 
   
   
       6 . The MEMS resonator of  claim 2 , wherein an outside sidewall of the at least one resonating element has a serrated surface. 
   
   
       7 . The MEMS resonator of  claim 6 , wherein the serrated surface comprises teeth. 
   
   
       8 . The MEMS resonator of  claim 7 , wherein the at least one resonating element is configured to oscillate in a tuning fork fashion during operation, and a region proximate to the base of the teeth is the region subject to the relatively larger stresses. 
   
   
       9 . The MEMS resonator of  claim 6 , wherein the serrated surface comprises an irregular profile. 
   
   
       10 . The MEMS resonator of  claim 2 , wherein the at least one resonating element is configured to oscillate in an extensional fashion during operation, and a region proximate to where the at least one resonating element connects to the base element is the region subject to the relatively larger stresses. 
   
   
       11 . The MEMS resonator of  claim 1 , wherein the one or more slots are filled completely with the compensating material. 
   
   
       12 . The MEMS resonator of  claim 1 , wherein the one or more slots are partially filled with the compensating material to produce a gap within each of the one or more slots. 
   
   
       13 . The MEMS resonator of  claim 12 , wherein the gaps in each of the one or more slots are filled with a low-stress cap layer. 
   
   
       14 . The MEMS resonator of  claim 13 , wherein the low-stress cap layer comprises silicon. 
   
   
       15 . The MEMS resonator of  claim 1 , wherein the one or more slots are lined with a liner material resistant to an etch process. 
   
   
       16 . The MEMS resonator of  claim 15 , wherein the liner material comprises silicon. 
   
   
       17 . The MEMS resonator of  claim 15 , wherein the one or more slots are capped with a conductive capping material resistant to the etch process. 
   
   
       18 . The MEMS resonator of  claim 17 , wherein the conductive capping material comprises silicon. 
   
   
       19 . The MEMS resonator of  claim 17 , wherein the one or more slots are filled completely with the compensating material. 
   
   
       20 . The MEMS resonator of  claim 17 , wherein the one or more slots are partially filled with the compensating material to produce a gap within each of the one or more slots. 
   
   
       21 . The MEMS resonator of  claim 20 , wherein the gap in each of the one or more slots is filled with a low-stress cap layer. 
   
   
       22 . The MEMS resonator of  claim 21 , wherein the low-stress cap layer comprises silicon. 
   
   
       23 . The MEMS resonator of  claim 1 , wherein one or more slots are also included in the base element. 
   
   
       24 . The MEMS resonator of  claim 1 , wherein the compensating material comprises oxide. 
   
   
       25 . An electronic device, comprising:
 application circuitry; and   a microelectromechanical system (MEMS) resonator for generating a reference signal for the application circuitry, the MEMS resonator comprising:   a base element, and   at least one resonating element coupled to the base element that includes one or more slots filled with a compensating material,   wherein a temperature coefficient of Young's Modulus (TCE) of the compensating material has a sign opposite to a TCE of a material comprising the at least one resonating element.   
   
   
       26 . The electronic device of  claim 25 , wherein the one or more slots are positioned in a region of the at least one resonating element subject to larger stresses relative to other regions of the at least one resonating element.